1. Field of the Invention
This invention relates generally to a power steering apparatus which increases steering power with the assistance of an electric motor and, in particular, to a power steering apparatus provided with a compact driving device comprising a small-sized motor so as to increase steering-assisting power in proportion to increase in steering angle.
2. Prior Art
There has been developed an electric power steering apparatus in which a motor for increasing steering-assisting power is driven on the basis of result of sensing steering torque exerted on the steering wheel and power required for operating the steering wheel is intensified by rotational power of the abovesaid motor for making the driver to feel comfortable in steering.
In the automobile equipped with a rack-pinion type steering mechanism composed of a steering shaft having a steering wheel fitted on one end thereof, a pinion shaft connected to the other end of the steering shaft through universal joints, and a rack shaft in the gearing relation with the pinion shaft and extending across the care body so that steering is performed by converting the rotation of the pinion shaft caused by the turning operation of the steering wheel into the axial displacement of the rack shaft, the aforesaid power steering apparatus is in such structure that the pinion shaft is divided into an input shaft connected to the steering shaft and an output shaft connected to the rack shaft, and a torsion bar is interposed between these two shafts for connecting them together on the same axial line, for example, a structure comprising a potentiometer which outputs electric potential corresponding to relative positions of both shafts along the circumferential directions thereof for sensing steering torque exerted on the steering wheel while depending on variation in potential outputted from the potentiometer as relative displacement between the input shaft and output one caused by torsion in the torsion bar corresponding to steering torque, and a steering-assisting motor arranged for driving rotational power thereof through an appropriate reduction gear device to the output side of the abovesaid pinion shaft or another pinion shaft being in gearing engagement with the rack shaft at a position along the axial direction different from that of the aforesaid pinion shaft.
The conventional power steering apparatus of such structure as above is followed by disadvantages that, because of both the torque sensor comprising the potentiometer and the steering-assisting motor disposed near the pinion shaft and rack shaft which are positioned low in the engine room having less spare space, a hard work in a limited space is inevitable in repairing or adjusting when any trouble is caused in the torque sensor or motor and, in addition, comparatively large sizes of the motor and reduction gear device possibly make it difficult to obtain setting positions thereof, particularly, in the case of the minicar.
Power required for steering the car varies according to the running speed of the car, increasing to a comparatively high degree during low speed running whereas decreasing to a slight degree during high speed running. Accordingly, a steering-assisting motor capable of enough reducing steering power during low speed running and halt of the car, when used in such power steering apparatus as above, is obliged to be large-sized, makes arrangement thereof more difficult, and causes excessive steering-assisting power during high speed running, whereby the steering wheel vibrates even with a slight degree of force applied thereto and possibly reduces stability in straightforward driving. Such disadvantages can be eliminated by providing a structure to control driving electric current of the motor and to vary rotational power of the motor according to the driving speed of the car or the turning angle of the steering wheel, or using a motor of small capacity in consideration of an appropriate stiffness to be endowed to the steering wheel during high speed running, however, in the former case, the structure is complicated at a control part and possibly increases frequency of trouble occurrence as well as, in the latter case, satisfactory steering-assisting power cannot be obtained at the time of halt and low speed driving, thereby requiring a large degree of force for steering operation.